Calculate days from filter, overlapping date ranges

Hi mhortman,

According to your description, you want to get the datediff in the selected data range, right?

If this is a case, you can refer to below formulas to achieve your requirement.

Logic :
1. If selected start date larger than current start date, use selected start date to calculate.
2. If selected end date less than current end date, use selected end date to calculate.
3. If selected start date large then current end date or selected end date less than current start date, remove.

1. Use start date and end date to build a calendar table.

CALENDAR = CALENDAR(FIRSTDATE('Table'[StartDate]),LASTDATE('Table'[EndDate])) 

2. Write a measure to filter range and calculate the dynamic date.

Diff = 
var start_Date=FIRSTDATE(ALLSELECTED('CALENDAR'[Date]))
var end_Date=LASTDATE(ALLSELECTED('CALENDAR'[Date]))
var current_Start=MAX('Table'[StartDate])
var current_end=MAX('Table'[EndDate])
return
IF(current_end>start_Date&&current_Start<end_Date,DATEDIFF(MAX(start_Date,current_Start),MIN(end_Date,current_end),DAY),BLANK())

3. Add measure to display the calculated date range.

calculate_start = 
var start_Date=FIRSTDATE(ALLSELECTED('CALENDAR'[Date]))
var current_Start=MAX('Table'[StartDate])
return
IF([Diff]<>BLANK(),MAX(current_Start,start_Date))

calculate_end = 
var end_Date=LASTDATE(ALLSELECTED('CALENDAR'[Date]))
var current_end=MAX('Table'[EndDate])
return
IF([Diff]<>BLANK(),MIN(current_end,end_Date))

If above not help, please share some detail contents.

Regards,

Xiaoxin Sheng